In light of the recent social demand for saving energy and resources, rubber materials with high breaking resistance, high abrasion resistance, and high crack growth resistance are now desired commonly to respond to the demand for tires with improved durability. Further, a surge in natural rubber prices has created a need for developing synthetic rubber that exhibits durability equal to that of natural rubber.
To enhance durability of the synthetic rubber, the cis content of synthetic polyisoprene is conventionally increased to improve strain-induced crystallinity. Refer, for example, to JP 2004-27179 A (PTL 1) and WO 2006/078021 (PTL 2). However, the synthetic rubber improved in this manner is still less durable than natural rubber when subjected to higher severity conditions.
Furthermore, a polymer having an isoprene skeleton is likely to show main-chain breaks compared to a polymer composed of other monomers. This main-chain break is considered causing the less durability of the polymer under high severity conditions. Additionally, in synthesizing a polymer to impart high-molecular weight, the chain ends of the polymer may be denatured with tin tetrachloride or titanium tetrachloride. This denaturation involves gelation commonly, thereby significantly reducing the durability of the polymer.